THE LOCALISATION OF BREAKS AND FAULTS. 471 



the Hue current (c) ; and as these two currents are to be equal, 

 we have 



y 



Whence 



f+y 

 f fu 



no f+y f{f + yy 

 Now f=F-x 



and JlL^Y.-x, 



f+y 



therefore c = 7ic ( ^ — '- ) • 



\Jb -x/ 



This gives the relation between the line currents observed 

 on the milammeter for equal currents through the fault in the 

 free and earth readings. 



As before, a preliminary test is made to get an approximate 

 value of a; by formula (1). The earth reading should then be 

 taken with any convenient current (iic) milliamperes to line, 

 this current being observed on the milammeter. The free 

 reading is then taken, adjusting the line current (as shown on 

 the milammeter) to 



-(1^:) (2) 



This gives a nearer approximation ioxx. Then applying the 

 new values of F and .vin formula (2) and making any necessary 

 modification in the line current accordingly, another pair of 

 observations is taken. After a few pairs taken in this way there 

 will be little or no further change to make in the adjustment 

 of the line current and the free and earth readings may then 

 be taken as correct and the value of x finally worked out by 

 formula (1). 



Clark's Method for Localisation of Fault by Fall of Foten - 

 tial. — This test for the localisation of high resistance faults 

 was devised by Mr. Latimer Clark and described in his 

 "Elementary Treatise on Electrical Measurement," of 1863, 

 p. 129. There are three measurements of potential to be made, 

 two at one end and one at the other. As the usual methods 

 involved reading deflections or throws, it was customary to 



